1. Field of the Invention
The present invention relates to diversity reception techniques for radio broadcast receivers.
2. Description of the Related Art
The well-known diversity reception techniques for reducing multipath interference or the like in radio broadcast receivers include a selection diversity scheme which selects and uses a signal in a good reception condition from among signals respectively received by a plurality of antennas provided in the receiver, as disclosed in, for example, JP-A-7-283767, and an in-phase combination type diversity scheme which combines in phase a plurality of signals respectively received by the antennas provided in the receiver, as disclosed in, for example, JP-A-5-235812.
The in-phase combination type diversity can generally reduce multipath interference more effectively than the selection diversity. In some cases, however, even the in-phase combination type diversity cannot completely reduce the multipath interference. For example, suppose that two antennas respectively receive two wave signals having passed through different paths at the same level, and that the first wave signal having traversed the first path reaches the two antennas in phase, while the second wave signal having traversed the second path reaches the two antennas at different times by a difference in phase angle X.
FIG. 4(a) illustrates a relationship between a difference in phase angle X and a distortion ratio of a reception signal received by the in-phase combination type diversity scheme.
As shown in the figure, in the in-phase combination type diversity scheme, when the phase-angle difference X is large, the distortion ratio of the received signal can be reduced to a smaller value, while, when the phase-angle difference X is small, the in-phase combination type diversity scheme does not function effectively, and the distortion ratio cannot be reduced to the smaller value.
FIG. 4(b) illustrates a relationship between a distortion ratio of a reception signal received by the in-phase combination type diversity scheme, and a level ratio of a reception signal received by the first antenna to that received by the second antenna when the first wave signal having traversed the first path reaches in phase the two antennas, and the second wave signal having traversed the second path reaches the two antennas at different times by a phase-angle difference of 60 degrees. In the in-phase combination type diversity scheme, when the level of the received signal at the first antenna is approximately the same as that of the received signal at the second antenna, the distortion ratio of the received signal can be reduced to a smaller value, but when a difference in level of the received signals between the two antennas is large, the scheme does not function effectively and the distortion ratio cannot be reduced to the smaller value.
Thus, the in-phase combination type diversity scheme has an area of operation where the multipath interference cannot be reduced effectively.